JP2765953B2 - Electrode for electrochemical detector - Google Patents

Description

Description: BACKGROUND OF THE INVENTION <Industrial Application Field> The present invention relates to an electrode for a coulometric electrochemical detector incorporated in an electrochemical detector. In particular, the present invention relates to an electrode for a coulometric electrochemical detector capable of detecting a trace amount of an electroactive substance separated by liquid chromatography or the like with high sensitivity.

<Prior art> Conventionally, an electrochemical detector incorporating an electrode for an electrochemical detector is combined with liquid chromatography to form a trace amount of an electrochemically active substance contained in a biological sample or the like. For example, since catecholamines and their metabolites can be detected with extremely high sensitivity, they are used for the detection and quantification of these electroactive substances.

Amperometric electrodes have been used as electrodes for these electrochemical detectors, but they have low resolution and cannot be applied to more precise analysis. Further, as an electrode that can be detected with the highest sensitivity, an electrode used for a coulometric electrochemical detector is known.

It is desirable that the electrode for the coulometric type electrochemical detector is such that all of the sensing substance contained in the electrode cell is electrochemically converted and passed therethrough. Therefore, in order to react efficiently, a conductive material having a high surface area is required. The electrode surface has low polarity and is chemically stable because of its durability for long-term use, and the pore size distribution is constant to prevent clogging in the electrode cell It is required to have the performance of

It is known from U.S. Pat.No. 4,804,455 that it is preferable to use a carbon material, especially a carbon material in the form of graphite, which is low in polarity and chemically stable, as a conductive material that can satisfy the above conditions. Have been.

<Problems to be Solved by the Invention> However, the carbon material in the form of graphite introduced in US Pat. No. 4,804,455 has a problem that the surface area is remarkably small and the durability for long-term use is poor. This has given remarkable control especially on being widely used as a material for an electrode of a coulometric electrochemical detector.

[Summary of the Invention]

<Summary> The inventors of the present invention have conducted intensive studies in view of the above problems, and as a result, a porous graphitic carbon molded body having specific physical properties has a non-polar and stable chemical property on its surface. It has a moderately large surface area and a fixed pore size that does not cause clogging etc. even for long-term use, making it an electrode for electrical detectors, especially a coulometric type. The inventors have found that they can be used as electrodes for electrochemical detectors, and have completed the present invention.

That is, the electrode for a coulometric electrochemical detector of the present invention is characterized by being formed from a porous graphitic carbon molded body having the following physical properties (1) to (3). is there.

(1) The average pore size is 0.1 to 50 μm, (2) The specific surface area is 10 m ² / g or more, (3) The average carbon layer interval determined by X-ray diffraction analysis (d ₀₀₂ ) Is 3.35 to 3.42Å.

<Effect> The electrode for a coulometric electrochemical detector of the present invention has a nonpolar and stable chemical property, a moderately large specific surface area, excellent electric conversion characteristics, and a moderate and constant pore size. Therefore, clogging and the like hardly occur even when used as an electrode of an electrochemical detector for a long time. Therefore, when used as an electrode of a coulometric type electrochemical detector of a liquid chromatography device, excellent electrical conversion characteristics can be maintained for a long time, so that a trace amount substance separated by liquid chromatography can be used for a long time. It can be detected with high sensitivity.

[Specific description of the invention] [I] Electrode for electrochemical detector <Physical properties> The electrode for a coulometric electrochemical detector of the present invention has an average pore diameter of 0.1 to 50 µm, preferably 1 to 30 on its surface.
μm pores are formed, the non-surface area is 10 m ² / g or more, preferably 50 m ² / g or more, and the average carbon layer interval (d ₀₀₂ ) determined by X-ray analysis is 3.35 to 3.42Å. Preferably 3.
It is formed from a porous graphitic carbon compact having physical properties of 35 to 3.40Å.

If the average pore diameter of the molded article is too small, if the molded article is used as an electrode for a long time, the pores are clogged with a detection substance or an impurity, and cannot be used as an electrode. On the other hand, if the average pore diameter is too large, the contact between the liquid and the inner surface of the electrode becomes insufficient, so that efficient analysis cannot be performed.

If the specific surface area is too small, sufficient electric conversion characteristics cannot be obtained, and efficient analysis cannot be performed. further,
Carbon materials with an average carbon layer spacing of greater than 3.42 mm are poorly graphitized and have a very chemically active surface, which can cause a chemical reaction with the detection substance, deteriorating the surface, Cannot withstand long-term use.

The average pore diameter can be determined by the mercury intrusion method, and the (d ₀₀₂ ) average layer spacing of carbon can be determined by X-ray diffraction and Bragg's equation.

[II] Production The porous graphitic carbon electrode of the present invention can be produced, for example, by the following method.

That is, a composition obtained by mixing a carbon fiber obtained by a gas phase method with a specific surface area of 10 m ² / g or more and a medium containing a carbonizable binder for binding the carbon fiber and a pore formation promoter, After molding into a desired shape by molding or the like, the target object can be manufactured by a method of firing in a non-oxidized state.

<Carbon Fiber Obtained by Gas-Phase Method> As the carbon fiber obtained by the gas-phase method used in the above method, a carbon source compound such as hydrocarbon or carbon monoxide may be used as a catalyst for ultrafine fiber metal catalyst such as Fe or Co. It is a fibrous graphitic carbon material obtained by pyrolyzing carbon in the presence and growing carbon in the gas phase, or obtained by heating this.
Since carbon fibers other than the carbon fibers obtained by the vapor phase method are slightly inferior in crystallinity, it is preferable to use carbon fibers obtained by the vapor phase method.

The specific surface area of the carbon fiber obtained by the gas phase method is 10 m ² / g or more, preferably 50 m ² / g or more as measured by the BET method. The upper limit is not particularly limited, but is preferably about 400 m ² / g.

As a method for producing a carbon fiber obtained by a suitable high specific surface area (50 m ² / g or more) gas phase method, for example, a method described in Japanese Patent Application No. 63-322431 is described in detail. Can be adopted.

Specifically, a carbon source compound is supplied together with a carrier gas to a heating zone in a reaction tube, and a transition metal carbonyl compound is subjected to gas phase pyrolysis in the presence of at least one organic compound containing oxygen and / or sulfur. It can be produced by a method of producing graphite whiskers by decomposing the carbon source compound in a gas phase space in the presence of a catalyst comprising the obtained transition metal ultrafine particles.

The diameter and length of the carbon fiber obtained by the gas phase method are usually 5 μm or less, preferably 0.01 to 2 μm.
It is suitable to use one having a fiber length of 1 μm or less, preferably 1 to 500 μm.

<Carbonizable binder> The carbonizable binder binds the carbon fibers obtained by the gas phase method to each other during mixing and granulation. Examples of such a carbonizable binder include thermosetting resins such as phenolic resins (such as resol resins and novolak resins), furan resins, acrylic resins, epoxy resins, and unsaturated polyester resins.
Thermoplastic resins such as ABS resin, polycarbonate resin, and polypropylene resin can be used. Also, pitch, tar, and the like can be used. Among them, it is preferable to use a phenol resin having a relatively high carbonization yield, particularly a liquid phenol resin or pitch.

<Diluting medium> Examples of the diluting medium used for forming the carbonizable binder into a solution or a dispersion include water, methanol,
Tetrahydrofuran, pyridine, quinoline, benzene,
Examples include toluene.

The concentration of the carbonizable binder in the diluting medium is determined based on the type of the carbonizable binder, the type of the diluting medium including the solvent or the dispersion medium, and the carbon fiber obtained by the gas phase method mixed with the diluting medium. It varies depending on the surface area, the average fiber length, the porosity of the porous granular material to be obtained, the mechanical properties and the like, but is usually in the range of 5 to 60% by weight, preferably 10 to 30% by weight. It is usually done in.

<Pore Formation Accelerator> The pore formation promoter used for the purpose of providing porous graphite having pores having a sharp pore size distribution as a graphitic molded article is decomposed by firing in a non-oxidizing atmosphere. A substance which is gasified and hardly remains as carbon can be used. Examples include starch, cellulose, polyethylene oxide and polyvinyl alcohol. The addition of the pore formation promoter can be carried out in a diluting medium comprising a solution or a dispersion medium of the above-mentioned carbonizable binder.

An appropriate amount of the pore formation promoter is added to the solution or dispersion medium of the carbonizable binder material described above so that the average pore size at the time of forming the porous graphite molded product is in the range of 0.1 to 50 μm.

<Firing> The composition obtained as described above is dried, pressed into an arbitrary shape by a molding press, and then heated. By this heating, the carbonizable substance as a binder is carbonized to be graphitized, and the pore formation promoter is decomposed to form porous graphite having fine pores having a sharp pore size distribution. The heating is performed in a non-oxidizing inert gas such as nitrogen or argon, and the heating temperature is 800 to 3000 ° C, preferably 1000 to 2 ° C.
It is usually in the range of 500 ° C. and the heating time is usually in the range of 5 to 20 hours.

<Porous Graphitic Carbon> The porous graphite carbon obtained by the above method has an average pore size of 0.1 to 50 μm, preferably 1
Many pores having a specific surface area of 10 m ² are formed.
/ g or more, preferably 50 m ² / g or more. Further, when this was measured by X-ray analysis, the average layer interval (d ₀₀₂ ) of carbon was 3.35 to 3.42 °, indicating that the carbonaceous material was graphite.

[III] Use as Electrode for Electrochemical Detector This porous graphitic carbon molded body is formed into a desired shape as an electrode for a detector by cutting or the like, and then electrochemically detected as an electrode for a coulometric electrochemical detector. It is used by being built into a vessel. In particular, it is suitable for detecting and quantifying trace substances in a living body in combination with high performance liquid chromatography (HPLC).

(Experimental example)

Example 1 <Production of molded article> 3 g of a commercially available water-soluble phenol resin having a solid content of 65% by weight
Was added to 3 liters of water and heated. Next, a carbon fiber (specific surface area 130 m ² /
g, average diameter 0.2 μm, average fiber length 2 μm), 100 g, potato starch 10 g and sodium alginate 0.2 g were added and mixed. This mixed solution was dripped little by little into a 10% aqueous solution of calcium chloride from a nozzle to form a spherical gelled liquid of the dripped liquid.

The obtained spherical particles were filtered and then dried at room temperature.

Next, the spherical particles were placed in a stainless steel mold having a diameter of 45 mm, molded under pressure at a pressure of 0.5 kg / cm ² , and then cured by heating at a temperature of 200 ° C. for 2 hours to obtain a cured molded body. The molded body is heated up to a temperature of 1000 ° C. at a heating rate of 70 ° C./hr, kept at a temperature of 1000 ° C. for 6 hours, then heated again, and further heated at a temperature of 2000 ° C. for 15 minutes. The product was obtained by heating and holding and cooling.

This product has a specific surface area of 51 m ² / g, an average pore size of 3 μm, a sharp distribution, and a graphite structure with an average layer spacing (d ₀₀₂ ) of 3.38% by X-ray diffraction. It was shown.

<Electrochemical detector> The product having the above graphite structure was cut into an electrode for an electrochemical detector having a diameter of 5 mm x 3 mm, and this electrode was incorporated into an electrochemical detector using Ag / AgCl as a reference electrode. The following catecholamines were analyzed in combination with high performance liquid chromatography.

 The results obtained are shown below.

Phosphate buffer (pH 3.3) / methanol / n as mobile phase
An aqueous solution in which norepinephrine, epinephrine; dihydroxybenzoic acid (an internal standard substance) and dopamine are dissolved is injected into a separation column filled with octadecylated silica gel in which a mixture of sodium octyl sulfate / EDTA is flowing, and an eluent is used. Was passed through the electrochemical detector to obtain a chromatogram.

As a result, the electrochemical oxidation rate is very high (99% or more) in the electrochemical cell equipped with the electrode according to the present invention.
A chromatogram as shown below was obtained even with a very small amount of 20 picograms.

The detection limit of catecholamines and their metabolites by this detector was 1 picogram, which was 500 to 1000 times higher than the conventional electrochemical detector.

[Brief description of the drawings]

FIG. 1 shows an analysis chart according to the embodiment of the present invention. 1 ... norepinephrine peak, 2 ... epinephrine peak, 3 ... dihydroxybenzoic acid (internal standard substance)
Peak, 4... Dopamine peak.

──────────────────────────────────────────────────続 き Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 27/30

Claims (1)

(57) [Claims] 1. An electrode for a coulometric electrochemical detector, which is formed from a porous graphitic carbon molded body having the following physical properties (1) to (3). (1) The average pore size is 0.1 to 50 μm, (2) The specific surface area is 10 m ² / g or more, (3) The average carbon layer interval determined by X-ray diffraction analysis (d ₀₀₂ ) Is 3.35 to 3.42A.